The sulfate or kraft process is the dominant alkaline pulping technique used in the pulp and paper industry. The spent liquor resulting from the pulping process is commonly known as black liquor. It contains various organic materials as well as the inorganic pulping chemicals in an aqueous medium. The typical contents of black liquor are: dissolved wood substances (polymeric lignin and various aromatic organic compounds); carbohydrate derivatives (cellulose and hemicellulose degradation products); extracted light organic compounds (fatty and resinous acids); and inorganic chemicals from cooking liquor (sodium hydroxide, sodium sulphide, sodium carbonate, sodium sulphite and sodium chloride). The black liquor is concentrated by evaporation and fired in a boiler.
It is well established that the regeneration of the black liquor to fresh white liquor is an economically and environmentally necessary part of the kraft process. The recovery of the pulping chemicals is obtained by incinerating the black liquor in a chemical recovery boiler. The chemical recovery boiler is perhaps the single most important unit in a kraft process. In the boiler, the organic matter in the black liquor is combusted hence generating process heat in the form of steam. Meanwhile, an inorganic smelt is also produced, recovered from the boiler and dissolved to yield what is known as green liquor. The green liquor is then treated by causticizing it with slaked lime. The resultant solution of fresh pulping chemicals also known as white liquor can be used again in the digester unit. Maximum recovery of heat and pulping chemicals and the minimum generation of polluting wastes are crucial to the overall pulping process.
It is well established that energy saving, enhancement of the chemical recovery boiler throughput capacity, and abatement of pollution, notably SO.sub.2 and H.sub.2 S flue gas emissions, are improved by incinerating a more concentrated black liquor in the recovery boiler. Such concentrated black liquors are known as high solids content black liquor. Their concentration is usually expressed as a weight percentage of solids content. To illustrate the importance of higher solids concentration in black liquor, it has been shown that if the solids content were raised from 60% to 80% before firing, the energy saving would be about 760.times.10.sup.9 J/day for a typical 1000 ton/day mill. Consequently, there is a continuous trend toward firing black liquor at higher solids concentrations in the recovery boiler.
However, the single most important drawback to firing high solids content black liquor relates to its viscosity which dramatically increases with its solids content. The more the black liquor is concentrated in evaporators and the like, the more it tends to cause plugging of the evaporators, concentrators, transport lines, and the boiler firing nozzles. Consequently, although it is possible to currently obtain solids content of about 85%, black liquor solids content is generally maintained at approximately 60-65% to avoid plugging and fouling of equipment.
Various attempts have been made to modify the evaporators, transport lines and nozzles to permit the handling of higher solids content black liquor. However, these attempts have gained little acceptance in view of the substantial capital investments and technical difficulties.
To avoid large capital expenses and equipment modifications, viscosity reducing additives to the black liquor have been proposed. Notably, U.S. Pat. No. 4,776,889 teaches the addition of waxes as viscosity reducing agents and U.S. Pat. No. 4,734,103 teaches the addition of a water soluble terpolymer composition. Japanese Patent 59228094 teaches the addition of high molecular weight poly-ether diols made from mixtures of polypropylene glycol and polyether oxide or propylene oxide. However, these techniques have also met with limited market acceptance.
Consequently, there remains an important need for the discovery of improved viscosity reducing additives.
It is therefore an object of the present invention to provide a black liquor comprising a viscosity reducing additive which will allow the firing of black liquors having solids content higher than 65% wt.
It is another object of the present invention to provide a viscosity reducing additive which will improve the overall economic feasibility, tend to diminish the obnoxious wastes, and tend to improve the firing characteristics of the chemical recovery boiler operation.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter.
It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.